While returning from an island sightseeing flight about 25 miles into the Pacific Ocean, the pilot of the rented vintage airplane heard what he thought was a change in engine sound. About five minutes later, while cruising at about 2,400 rpm, with oil pressure and oil temperature indicating normal, the engine suddenly stopped producing power. Although the wood propeller continued to windmill until the airplane was slowed to about 60 miles per hour, the pilot could not get the engine restarted and ultimately ditched in the ocean. The duration of the flight was 1.6 hours and the pilot reported that the 13-gallon fuel tank appeared full prior to engine start. A subsequent records review revealed that the airplane had been flown for .6 of an hour on the previous day, and had not subsequently been refueled. According to the certified flight instructor, who checked the pilot out in the airplane, the pilot was informed that due to its limited fuel quantity, when fully fueled, the airplane needed to be back at the airport two hours after departure. The total time logged since the last refueling was 2.2 hours, and the flight to the island included an outbound climb to about 5,000 feet.